Update API to have a sensible grouping of sig_refs rather than separate arrays of sig_refs, sig_refs_names and sig_refs_urls

data/sig_ref.py

@@ -0,0 +1,12 @@
+from dataclasses import dataclass
+
+# Data class that defines a Special Interest Group "info" or reference. As well as the basic reference ID we include a
+# name and a lookup URL.
+@dataclass
+class SIGRef:
+    # Reference ID, e.g. "GB-0001".
+    id: str
+    # Name of the reference, e.g. "Null Country Park", if known.
+    name: str = None
+    # URL to look up more information about the reference, if known.
+    url: str = None